The present invention relates generally to the field of temperature measurement and thermocouple devices. More particularly, the invention relates to a novel technique for effectively determining thermocouple cold junction temperature in a multi-channel terminal configuration.
Thermocouples are devices used to measure temperature and are one of the more versatile temperature sensors available. These temperature sensors or transducers are generally rugged and relatively inexpensive, and may be constructed of various metals. Thermocouples may be used to measure a relatively wide range of temperatures (e.g., −200° C. to 2600° C.) in a variety of applications and environments. In general, thermocouples rely on the principle that a voltage potential occurs when there is a temperature gradient alone the length of a conductor.
A thermocouple device is formed by joining two conductors or wires of dissimilar metals to form a junction of the two wires called a measuring junction (or sensing junction, and so on). Although almost any two types of metal can be used to make the thermocouple wires, a number of standard types are used because they possess predictable output voltages and large temperature gradients. The several types of thermocouples available may be designated by capital letters that indicate their composition according to American National Standards Institute (ANSI) conventions. For example, a J-type thermocouple has one iron conductor and one constantan (copper-nickel alloy) conductor.
The thermocouple measuring junction may be encased in a sensor probe, for example, with the probe positioned at the point of temperature measurement (i.e., at the temperature source). In principle, as the temperature of the measuring junction changes with the temperature source, a temperature gradient is formed (along the wires) between the measuring junction and the opposite free ends of the two wires. Advantageously, a predictable thermoelectic voltage is generated as a function of this temperature gradient. By taking into account the composition of the two dissimilar metal wires, this generated thermoelectric voltage (sometimes called the “Seebeck” voltage) can be related to the temperature gradient along the wires. This temperature gradient is summed with a reference or “cold” junction temperature to give the temperature of the source being measured. Thermocouple measurements typically require sensing or determining the reference temperature (the cold junction temperature) where the thermocouple wires connect to the voltage measurement system.
In construction, the two free ends of the wires may be connected at a voltage measuring instrument (e.g., an analog to digital instrument, voltmeter, control instrument, terminal block, terminal strip, temperature control module etc.) to measure the thermoelectic voltage. The connection of these two wires at a terminal block, for example, forms the second junction of the wires called the reference junction (or cold junction). The term “cold junction” comes from the traditional practice of holding this reference junction at zero degrees Celsius in an ice bath. However, maintaining an ice bath is not practical for most measurement applications. Thus, the actual temperature of the point of connection of the thermocouple wires to the measuring instrument is measured and recorded. Typically cold junction temperature may be sensed by a thermistor or other temperature sensor or device, which is in relatively good thermal contact with the input connectors of the measuring instrument. Again, this second temperature reading, the cold junction temperature, is used by the measuring instrument to calculate the true temperature at the thermocouple tip, the measuring junction.
In sum, to determine the measured temperature of the source (i.e., the component, process, system, equipment, etc.) at the measuring junction of the two wires, the calculated temperature gradient based on the measured voltage is summed with the cold junction temperature (e.g., measured with a thermistor) at the voltage measuring instrument or terminal block. Thus, in operation, a thermocouple measures temperature by generating a voltage (e.g., millivolt) proportional to the temperature difference between the measuring and cold junctions of two dissimilar metals. For smaller changes in temperature, the voltage is substantially linearly proportional to temperature difference. For relatively larger changes in temperature, the relationship may become non-linear.
In ever-increasing demanding applications in precision temperature measurement, and with equally-demanding desires to reduce costs, the cold junction temperature measurement can be problematic. In a variety of configurations, errors in the measurement of the cold junction temperature give errors in the measurement of the source temperature. A typically accurate but expensive technique is to use a high-accuracy thermistor affixed to each terminal or to the channel that contains the two terminals. A less expensive but also less accurate solution is a single integrated circuit sensor. A particular problem is with multi-channel terminal blocks that may be a strip of channels or tiers of channels, which may include up to 30 or more channels (60 terminals or terminations), for example, to receive wires from several thermocouples and other devices forming several cold junctions. Relatively significant temperature gradients may exist across the rows or steps of the multi-tier terminal block. There is a need, therefore, for cost-effective and accurate measurement of cold junction temperature in single row and multi-tier terminal blocks, which may have several input terminals on multiple rows. Mounting a single temperature sensor (i.e. thermistor) on each row may not be adequate because of a change in the temperature profile along the terminal row (tier, strip, etc.). A cost-prohibited option is to mount a thermistor at each terminal or channel. For multi-tier or multi-channel terminal blocks and other termination devices, there is a need to provide for an efficient, cost-effective, accurate, and precise temperature measurement of the cold junction (i.e., reference junction).